The recovery of minerals from beneath the ground dates from prehistoric times. For thousands of years mining consisted of the excavation of outcropping material or tunneling more or less horizontally into mountainsides. Mining from deep shafts became possible only when reliable supports for tunnels along with drainage, ventilation and the use of mechanical appliances were developed.
Methods for the mining of mineral deposits differ from one another because of differences in geological conditions and location. However, all underground mines have certain features in common. Access to the deposit is gained either by a horizontal tunnel driven into a mountainside, a diagonal shaft to access the deposits, or by a veltical shaft. The deposit is then generally divided into sections of suitable size and shape for mining. When the mineral deposit extends to great depths as is common, for example, in coal mines, multiple horizontal tunnels may be formed from a vertical shaft, sometimes spaced from each other at intervals of as much as 300 feet. After extensive preparatoly work has been done. the actual mining may commence.
One well established method for the mining of sedimentary deposits occurring, in coal seams is known as "long wall mining". By this method, coal is obtained from a continuous wall of up to 200 yards long or longer by removal of a web of coal about 3 feet wide by the seam height. Areas of several hundred acres may be completely extracted.
Two variants of this method are in use. In the "retreating" system, roads are driven to the boundaries of the area to be mined. The faces are worked retreating to the panel heading. In the "advancing" system, the faces are opened up at the panel heading and then advanced to the boundaries.
In the United States the "room-and-pillar" method of coal mining is extensively used. Main entries are first made and from them rooms are driven. Between 30 and 50% of the coal is mined in this way during the "first working". Subsequently, the pillars of coal remaining are mined in the retreat, or "second working".
With either system, the passageways formed during the mining operation provide another critical function, namely, they enable ventilation of the mine. Ventilation in a mine is important for three main purposes: to provide fresh air to the miners, to dilute, render harmless and carry away, any potentially hazardous gases, dust, smoke and fumes that may be underground and, since underground temperature rises with increasing dental on an average of about 1.degree. C. for every 30 m (100 ft.), the ventilating air lowers the naturally occurring heat of the rock.
Horizontal-tunnel mining associated with hard mineral extraction usually relies on natural ventilation by utilizing a difference in air pressure between openings at different levels of the mine. In deep coal mining, however, it is generally necessary to use fans of very large size, drawing, perhaps, 20,000 m.sup.3 (700,000 ft..sup.3) of air per minute, installed at air-extraction shafts at an edge of the mined area in order to provide adequate ventilation.
The fresh air descends by negative pressure to the lowest levels of the mine and is heated or cooled by the natural heat of the rock. The ventilation air makes its way by various paths to the suction zone of the main extraction way or shaft, in which suction pressures of up to 400 mm (17 in.) water gauge may be maintained.
Parts of the mine that are not effectively serviced by forced ventilation may shave to be provided with an additional or auxiliary ventilation system. For this purpose air may be impelled by powerful fans through large-diameter ducts located in the mine. Proper functioning of this auxiliary system has to be supervised and controlled with considerable care and cost. Accordingly, use of auxiliary ventilation is minimized, where possible.
Thus, it will be recognized that ventilation air is a critical component of all underground mining operation. It is important that the air be provided to all portions of the mine in use; however, considering the cost of providing and circulating, the air. it is also commercially important to insure that ventilating air is not wasted.
Generally diagrams, including data on airflow conditions, are prepared for each section of the mine. For reasons of safety, the main air flow is split up into the largest possible number of circulating currents. It is essential to prevent "short circuits" which cause the air to take a shortcut and, thus, bypass certain parts of the mine. Distribution of the fresh air over the various levels, main roadways, crosscuts, rooms and workings is assisted by ventilation doors (designed as air locks), seals, stoppings, air crossings and other devices.
While such ventilation controls must be readily constructed in an inexpensive manner, they must still be substantially air tight and strong enough to prevent passage of high pressure air. Additionally, they must be relatively rigid, yet flexible enough to withstand significant pressure differential, on the order of 39 lb/ft.sup.2, and be flame retardant.
Currently, a stopping is commonly accomplished by building a mortarless cinder block wall and then coating the wall surface(s) with a sealant to provide strength and integrity to the wall, to add flame retardant insulation, and/or to reduce airflow, through the wall. Particularly useful sealant or coating materials for such mine stoppings, are described in U.S. Pat. Nos. 5,043,019 and 5,236,499, assigned to Sandvik Rock Tools. Inc. of Bristol, Va., ("Sandvik"), the disclosure of each of which is hereby incorporated herein in its entirety by reference. A preferred Sandvik sealant of a paste-like consistency adapted to be coated on the wall by hand is formed of about 1.8 to about 20% by weight of a water soluble silicate as a binder, about 3.6 to about 46% water as a diluent, about 0.01 to about 0.3% reinforcing filler fibers, up to about 48% clay as a texture filler, and about 1 to about 73% limestone to speed up the drying rate. With about 1.8 to 28% silicate, about 3.6 to about 50% water, about 0.08 to about 5% fibers, up to about 50% clay, and from about 1 to 73% limestone, the sealant may be provided in sprayable form.
A dry-stack concrete block wall constructed in this mariner can be made quite rigid by the sealant and possesses good flexural strength. However, construction of mine stoppings from concrete blocks or the like is labor intensive and, in many situations, quite difficult. The need to transport large numbers of concrete blocks to remote areas of an underground mine, in and of itself, is expensive and time consuming. Moreover, in many areas of a mine, the portion of the mine tunnel to be closed may be of limited height or cross-sectional area, requiring a miner to carry the relatively heavy concrete blocks some distance in a crouched position or even on his hands and knees.
Prior art attempts to overcome some of the burdens associated with concrete block mine stopping have included the use of filled polyethylene bags or even foamed (rigid) polystyrene sheets coated with sealant. Unfortunately, each of these approaches have bad little or no practical success.
Therefore, it is evident that there is a great need for an improved mine stopping, one that can be readily constructed, even in difficult locations, in a simple and expeditious manner. It is this need with which the instant invention is concerned.